Various kinds of dump vehicles are used in agriculture, construction, mining and other industries. In general, such vehicles are used to, at one location, receive materials such as produce, soil, ore, gravel, and construction debris for transport and off-loading to another site. A common example of such a vehicle is a dump truck. Other examples include dump trailers, sleds, hoppers and the like.
A typical dump truck includes a chassis with a dump bed mounted thereto so that the bed can receive materials to be transported while in a generally horizontal position. The dump bed is movably mounted to the chassis so that the materials received can subsequently be unloaded by rotating or tilting the bed, allowing the materials to flow out of the bed. Typically, such dump beds are hinge mounted with the hinge being located at a rearward portion of the chassis. The bed is tilted typically by a hydraulic cylinder located near a forward portion of the bed, causing the bed to rotate about the hinge from a horizontal position to a tilted position.
The hydraulic cylinder is pivotably mounted, directly or indirectly, to the chassis and to the dump bed so that as the cylinder is extended, the bed is pushed upward and pivots about its hinge. Such cylinders are generally powered by a PTO-driven pump which may be energized to pump oil from an onboard reservoir into the cylinder to cause it to extend, and a valving system is employed to permit oil to return to the reservoir as the cylinder retracts.
Due to the nature of the material typically transported in dump trucks and to the terrain over which they operate, it is common to have unbalanced loads experienced by cylinders used in dump trucks. These unbalanced loads give rise to unwanted bending moments which may severely affect the performance of the dump truck.
One example of an unbalanced load is evident when the load is not uniformly distributed within the bed. For example, when hauling debris, the debris may contain a mixture of soil, concrete, steel, framing materials, stumps, tree limbs, and leaves. A heavy concrete slab that winds up in a load disposed near one side wall of the dump bed, otherwise filled with tree limbs and relatively light debris, will cause a greater downward force on that side of the bed. Typically the cylinder is positioned near a transverse center of the bed and must therefore resist not only the vertical load but also the bending moment due to the concrete slab being disposed near one side wall of the bed.
Another example of an unbalanced load is the condition that arises due to unlevel terrain. Even where the material loaded in the dump bed is uniformly distributed and of uniform density, if the dump truck is on a transverse incline while dumping, side loads will occur due to gravity.
The bending moments generated by side loads in such situations are often very damaging to the hydraulic cylinder and to the pivotable connections of the cylinder to the dump bed and chassis. The bending moments generated by side loads will tend to bend the cylinder. In the least troublesome case, such cylinder bending may result in seal damage to the cylinder that then results in oil leakage and reduced performance. In the more catastrophic case, the bending moment may result in cylinder failure by breaking. Generally, in hydraulic cylinder design and selection, cylinders may be employed that are significantly over-designed to resist the side load moments, and this adds unwarranted cost to the dump vehicle.
Even with robust cylinder design to resist bending moments, there is the important issue of the pivotable connections of the cylinder to the bed and to the chassis or other support structure. Ultimately, it is these connections that must carry not only the axial load from the cylinder, but also the bending moment introduced by side loads. For example, the hydraulic cylinder used to actuate the dump vehicle typically includes a cylinder pivot bearing or sleeve that is journaled around a cross shaft mounted in a support structure. Side or lateral loads placed on the cylinder are transmitted to the bearing, and over time, these loads tend to distort the bearing, resulting in a loose connection with the cross shaft. This loose connection results in the dump bed being unstable as it is raised or supported by the hydraulic cylinder.